Bloomberg carried this article entitled Retirement Will Kill You. In summary, the author concludes that the academic studies show that people who retire die sooner than those who don't. In particular, retirees suffer from an increase in clinical depression, a decline in self assessed health and increased difficulties with mobility. At least one study looks at the question of whether underlying health issues can explain the observed deterioration in physical and mental health among retirees and concludes that "retirement doesn't harm health - and may actually improve it". Another contradicts this by finding "harm from early retirement".
Having looked at some of the studies (and some earlier ones), my take is that retirement is not, of itself, the issue.
If retirement means reduced physical activity, reduced mental stimulation, reduced social interaction and a loss of purpose, then it is not surprising that retirees' health deteriorates.
If retirement means walking away from a stressful job that requires long hours and impedes participation in other activities and embracing other activities (mental, physical and social) then it is difficult to see how or why the supposed causes of retirees' health problems would arise. At a personal level, I have mapped out the first two years in some detail (academic studies, writing a novel, one last trailwalker, marathon, volunteering, some travel) and have agreed to remain as a very part time consultant as a transitional matter. Hopefully these activities plus the day to day stuff will give me all the stimulation of the office, keep me physically, mentally and socially engaged and will result in reduced stress levels.